While much attention has been focused on high-level software
architectural patterns, what is, in effect, the de-facto standard
software architecture is seldom discussed. This paper examines this
most frequently deployed of software architectures: the BIG BALL OF MUD. A BIG BALL OF MUD is a casually, even
haphazardly, structured system. Its organization, if one can call it
that, is dictated more by expediency than design. Yet, its enduring
popularity cannot merely be indicative of a general disregard for
architecture.

These patterns explore the forces that encourage the emergence of a BIG BALL OF MUD, and the undeniable
effectiveness of this approach to software architecture. What are the
people who build them doing right? If more high-minded architectural
approaches are to compete, we must understand what the forces that
lead to a BIG BALL OF MUD are, and
examine alternative ways to resolve them.

A number of additional patterns emerge out of the BIG BALL OF MUD. We discuss them in
turn. Two principal questions underlie these patterns: Why are so many
existing systems architecturally undistinguished, and what can we do
to improve them?

Introduction

Over the last several years, a number of authors [Garlan & Shaw
1993] [Shaw 1996] [Buschmann et. al. 1996] [Meszaros 1997] have
presented patterns that characterize high-level software
architectures, such as PIPELINE and LAYERED ARCHITECTURE. In an ideal
world, every system would be an exemplar of one or more such
high-level patterns. Yet, this is not so. The architecture that
actually predominates in practice has yet to be discussed: the BIG BALL OF MUD.

A BIG BALL OF MUD is haphazardly
structured, sprawling, sloppy, duct-tape and bailing wire, spaghetticodejungle.
Weve all seen them. These systems show unmistakable signs of
unregulated growth, and repeated, expedient
repair. Information is shared promiscuously among distant elements of
the system, often to the point where nearly all the important
information becomes global or duplicated. The overall structure of the
system may never have been well defined. If it was, it may have eroded beyond
recognition. Programmers with a shred of architectural sensibility
shun these quagmires. Only those who are unconcerned about
architecture, and, perhaps, are comfortable with the inertia of the
day-to-day chore of patching the holes in these failing dikes, are
content to work on such systems.

Still, this approach endures and thrives. Why is this
architecture so popular? Is it as bad as it seems, or might it serve
as a way-station on the road to more enduring, elegant artifacts? What
forces drive good programmers to build ugly systems? Can we avoid
this? Should we? How can we make such systems better?

Why does a system become a BIG
BALL OF MUD? Sometimes, big, ugly systems emerge from THROWAWAY
CODE. THROWAWAY CODE is
quick-and-dirty code that was intended to be used only once and then
discarded. However, such code often takes on a life of its own,
despite casual structure and poor or non-existent documentation. It
works, so why fix it? When a related problem arises, the quickest way
to address it might be to expediently modify this working code, rather
than design a proper, general program from the ground up. Over time, a
simple throwaway program begets a BIG
BALL OF MUD.

Even systems with well-defined architectures are prone to structural
erosion. The relentless onslaught of changing requirements that any
successful system attracts can gradually undermine its structure.
Systems that were once tidy become overgrown as PIECEMEAL GROWTH gradually allows
elements of the system to sprawl in an uncontrolled fashion.

If such sprawl continues unabated, the structure of the system can
become so badly compromised that it must be abandoned. As with a
decaying neighborhood, a downward spiral ensues. Since the system
becomes harder and harder to understand, maintenance becomes more
expensive, and more difficult. Good programmers refuse to work there.
Investors withdraw their capital. And yet, as with neighborhoods,
there are ways to avoid, and even reverse, this sort of decline. As
with anything else in the universe, counteracting entropic forces
requires an investment of energy. Software gentrification
is no exception. The way to arrest entropy in software is to refactor
it. A sustained commitment to refactoring can keep a system from
subsiding into a BIG BALL OF MUD.

A major flood, fire, or war may require that a city be evacuated and
rebuilt from the ground up. More often, change takes place a building
or block at a time, while the city as a whole continues to function.
Once established, a strategy of KEEPING
IT WORKING preserves a municipalitys vitality as it grows.

Systems and their constituent elements evolve at different rates. As
they do, things that change quickly tend to become distinct from
things that change more slowly. The SHEARING
LAYERS that develop between them are like fault lines or facets that
help foster the emergence of enduring abstractions.

A simple way to begin to control decline is to cordon off the blighted
areas, and put an attractive faÁade around them. We call this strategy
SWEEPING IT UNDER THE RUG.
In more advanced cases, there may be no alternative but to tear
everything down and start over. When total RECONSTRUCTION becomes necessary,
all that is left to salvage is the patterns that underlie the
experience.

Some of these patterns might appear at first to be antipatterns
[Brown et al. 1998] or straw men, but they are not, at least in the
customary sense. Instead, they seek to examine the gap between what we
preach and what we practice.

Still, some of them may strike some readers as having a schizoid
quality about them. So, for the record, let us put our cards on the
table. We are in favor of good architecture.

Our ultimate agenda is to help drain these swamps. Where
possible, architectural decline should be prevented, arrested, or
reversed. We discuss ways of doing this. In severe cases,
architectural abominations may even need to be demolished.

At the same time, we seek not to cast blame upon those who must
wallow in these mires. In part, our attitude is to "hate the sin, but
love the sinner". But, it goes beyond this. Not every backyard storage
shack needs marble columns. There are significant forces that can
conspire to compel architecture to take a back seat to functionality,
particularly early in the evolution of a software artifact.
Opportunities and insights that can allow for architectural progress
often are present later rather than earlier in the lifecycle.

A certain amount of controlled chaos is natural during
construction, and can be tolerated, as long as you clean up after
yourself eventually. Even beyond this though, a complex system may be
an accurate reflection of our immature understanding of a complex
problem. The class of systems that we can build at all may be larger
than the class of systems we can build elegantly, at least at first. A
somewhat ramshackle rat's nest might be a state-of-the-art
architecture for a poorly understood domain. This should not be the
end of the story, though. As we gain more experience in such domains,
we should increasingly direct our energies to gleaning more enduring
architectural abstractions from them.

Indeed, to a substantial extent, much of this chapter describes
the disease, while the patterns above describe what we believe can be
the cure: a flexible, adaptive, feedback-driven development process in
which design and refactoring pervade the lifecycle of each artifact,
component, and framework, within and beyond the applications that
incubate them.

Forces

A number of forces can conspire to drive even the most architecturally
conscientious organizations to produce BIG
BALLS OF MUD. These pervasive, "global" forces are
at work in all the patterns presented. Among these forces:

Time: There may not be enough time to consider the long-term architectural
implications of ones design and implementation decisions. Even
when systems have been well designed, architectural concerns often
must yield to more pragmatic ones as a deadline starts to loom.

One reason that software architectures are so often mediocre is that
architecture frequently takes a back seat to more mundane concerns
such as cost, time-to-market, and programmer skill. Architecture is
often seen as a luxury or a frill, or the indulgent pursuit of
lily-gilding compulsives who have no concern for the bottom line.
Architecture is often treated with neglect, and even disdain. While
such attitudes are unfortunate, they are not hard to understand.
Architecture is a long-term concern. The concerns above have to be
addressed if a product is not to be stillborn in the marketplace,
while the benefits of good architecture are realized later in the
lifecycle, as frameworks mature, and reusable black-box components
emerge [Foote & Opdyke
1995].

Architecture can be looked upon as a Risk, that will consume
resources better directed at meeting a fleeting market window, or as
an Opportunity to lay the groundwork for a commanding advantage
down the road.

Indeed, an immature architecture can be an advantage in a
growing system because data and functionality can migrate to their
natural places in the system unencumbered by artificial architectural
constraints. Premature architecture can be more dangerous than none at
all, as unproved architectural hypotheses turn into straightjackets
that discourage evolution and experimentation.

Cost: Architecture is expensive, especially when a new domain
is being explored. Getting the system right seems like a pointless
luxury once the system is limping well enough to ship. An investment
in architecture usually does not pay off immediately. Indeed, if
architectural concerns delay a products market entry for too
long, then long-term concerns may be moot. Who benefits from an
investment in architecture, and when is a return on this investment
seen? Money spent on a quick-and-dirty project that allows an
immediate entry into the market may be better spent than money spent
on elaborate, speculative architectural fishing expedition. Its
hard to recover the value of your architectural assets if youve
long since gone bankrupt.

Programmers with the ability to discern and design quality
architectures are reputed to command a premium. These expenses must be
weighed against those of allowing an expensive system to slip into
premature decline and obsolescence. If you think good architecture is
expensive, try bad architecture.

Experience: Even when one has the time and inclination to take
architectural concerns into account, ones experience, or lack
thereof, with the domain can limit the degree of architectural
sophistication that can be brought to a system, particularly early in
its evolution. Some programmers flourish in environments where they
can discover and develop new abstractions, while others are more
comfortable in more constrained environments (for instance, Smalltalk
vs. Visual
Basic programmers.) Often, initial versions of a system are vehicles
whereby programmers learn what pieces must be brought into play to
solve a particular problem. Only after these are identified do the
architectural boundaries among parts of the system start to emerge.

Inexperience can take a number of guises. There is absolute,
fresh out of school inexperience. A good architect may lack domain
experience, or a domain expert who knows the code cold may not have
architectural experience.

Employee turnover can wreak havoc on an organizations
institutional memory, with the perhaps dubious consolation of bringing
fresh blood aboard.

Skill: Programmers differ in their levels of skill, as well as
in expertise, predisposition and temperament. Some programmers have a
passion for finding good abstractions, while some are skilled at
navigating the swamps of complex code left to them by others.
Programmers differ tremendously in their degrees of experience with
particular domains, and their capacities for adapting to new ones.
Programmers differ in their language and tool preferences and
experience as well.

Visibility: Buildings are tangible, physical structures. You
can look at a building. You can watch it being built. You can walk
inside it, and admire and critique its design.

A programs user interface presents the public face of a
program, much as a buildings exterior manifests its
architecture. However, unlike buildings, only the people who build a
program see how it looks inside.

Programs are made of bits. The manner in which we present these
bits greatly affects our sense of how they are put together. Some
designers prefer to see systems depicted using modeling languages or
PowerPoint pictures. Others prefer prose descriptions. Still others
prefer to see code. The fashion in which we present our architectures
affects our perceptions of whether they are good or bad, clear or
muddled, and elegant or muddy.

Indeed, one of the reasons that architecture is neglected is
that much of it is "under the hood", where nobody can see
it. If the system works, and it can be shipped, who cares what it
looks like on the inside?

Complexity: One reason for a muddled architecture is that
software often reflects the inherent complexity of the application
domain. This is what Brooks
called "essential complexity" [Brooks 1995]. In other words,
the software is ugly because the problem is ugly, or at least not well
understood. Frequently, the organization of the system reflects the
sprawl and history of the organization that built it (as per CONWAYS
LAW [Coplien 1995]) and the compromises that were made along the way.
Renegotiating these relationships is often difficult once the basic
boundaries among system elements are drawn. These relationships can
take on the immutable character of "site" boundaries that Brand [Brand
1994] observed in real cities. Big problems can arises when the needs
of the applications force unrestrained communication across these
boundaries. The system becomes a tangled mess, and what little
structure is there can erode further.

Change: Architecture is a hypothesis about the future that
holds that subsequent change will be confined to that part of the
design space encompassed by that architecture. Of course, the world
has a way of mocking our attempts to make such predictions by tossing
us the totally unexpected. A problem we might have been told was
definitely ruled out of consideration for all time may turn out to be
dear to the heart of a new client we never thought wed have.
Such changes may cut directly across the grain of fundamental
architectural decisions made in the light of the certainty that these
new contingencies could never arise. The "right" thing to do
might be to redesign the system. The more likely result is that the
architecture of the system will be expediently perturbed to address
the new requirements, with only passing regard for the effect of these
radical changes on the structure of the system.

Scale: Managing a large project is a qualitatively different
problem from managing a small one, just as leading a division of
infantry into battle is different from commanding a small special
forces team. Obviously, "divide and conquer" is, in general, an
insufficient answer to the problems posed by scale. Alan Kay, during
an invited talk at OOPSLA '86 observed that "good ideas don't always
scale." That observation prompted Henry
Lieberman to inquire "so what do we do, just scale the bad ones?"

BIG BALL OF MUD

alias

SHANTYTOWN

SPAGHETTI CODE

Shantytowns
are squalid, sprawling slums. Everyone seems to agree they are a bad
idea, but forces conspire to promote their emergence anyway. What is
it that they are doing right?

Shantytowns are usually built from common, inexpensive materials
and simple tools. Shantytowns can be built using relatively unskilled
labor. Even though the labor force is "unskilled" in the
customary sense, the construction and maintenance of this sort of
housing can be quite labor intensive. There is little specialization.
Each housing unit is constructed and maintained primarily by its
inhabitants, and each inhabitant must be a jack of all the necessary
trades. There is little concern for infrastructure, since
infrastructure requires coordination and capital, and specialized
resources, equipment, and skills. There is little overall planning or
regulation of growth. Shantytowns emerge where there is a need for
housing, a surplus of unskilled labor, and a dearth of capital
investment. Shantytowns fulfill an immediate, local need for housing
by bringing available resources to bear on the problem. Loftier
architectural goals are a luxury that has to wait.

Maintaining a shantytown is labor-intensive and requires a broad
range of skills. One must be able to improvise repairs with the
materials on-hand, and master tasks from roof repair to ad hoc
sanitation. However, there is little of the sort of skilled
specialization that one sees in a mature economy.

All too many of our software systems are, architecturally,
little more than shantytowns. Investment in tools and infrastructure
is too often inadequate. Tools are usually primitive, and
infrastructure such as libraries and frameworks, is undercapitalized.
Individual portions of the system grow unchecked, and the lack of
infrastructure and architecture allows problems in one part of the
system to erode and pollute adjacent portions. Deadlines loom like
monsoons, and architectural elegance seems unattainable.

vvv

As a system nears completion, its actual users may begin to work
with it for the first time. This experience may inspire changes to
data formats and the user interface that undermine architectural
decisions that had been thought to be settled. Also, as Brooks [Brooks
1995] has noted, because software is so flexible, it is often asked to
bear the burden of architectural compromises late in the development
cycle of hardware/software deliverables precisely because of its
flexibility.

This phenomenon is not unique to software. Stewart Brand
[Brand 1994] has observed that the period just prior to a
buildings initial occupancy can be a stressful period for both
architects and their clients. The money is running out, and the
finishing touches are being put on just those parts of the space that
will interact the most with its occupants. During this period, it can
become evident that certain wish-list items are not going to make it,
and that exotic experiments are not going to work. Compromise becomes
the "order of the day".

The time and money to chase perfection are seldom available, nor
should they be. To survive, we must do what it takes to get our
software working and out the door on time. Indeed, if a team completes
a project with time to spare, todays managers are likely to take
that as a sign to provide less time and money or fewer people the next
time around.

You need to deliver quality
software on time, and under budget.

Cost: Architecture is a long-term investment. It is easy for
the people who are paying the bills to dismiss it, unless there is
some tangible immediate benefit, such a tax write-off, or unless
surplus money and time happens to be available. Such is seldom the
case. More often, the customer needs something working by tomorrow.
Often, the people who control and manage the development process
simply do not regard architecture as a pressing concern. If
programmers know that workmanship is invisible, and managers don't
want to pay for it anyway, a vicious circle is born.

Skill: Ralph Johnson is fond of observing that is inevitable
that "on average, average organizations will have average people". One
reason for the popularity and success of BIG BALL OF MUD approaches might be
that this appoach doesn't require a hyperproductive virtuoso architect
at every keyboard.

It may seem to a programmer that whether to don hip boots and
wade into a swamp is a major quality-of-life matter, but programmer
comfort is but one concern to a manager, which can conflict with many
others. Architecture and code quality may strike management as frills
that have only an indirect impact on their bottom lines.

Therefore,
focus first on features and functionality, then focus on
architecture and performance.

The case made here resembles Gabriels "Worse is Better"
arguments [Gabriel 1991] in a number of respects. Why does so much
software, despite the best intentions and efforts of developers, turn
into BIG BALLS OF MUD? Why do
slash-and-burn tactics drive out elegance? Does bad architecture drive
out good architecture?

What does this muddy code look like to the programmers in the
trenches who must confront it? Data structures may be haphazardly
constructed, or even next to non-existent. Everything talks to
everything else. Every shred of important state data may be global.
There are those who might construe this as a sort of blackboard
approach [Buschmann 1996], but it more closely resembles a grab bag of
undifferentiated state. Where state information is compartmentalized,
it may be passed promiscuously about though Byzantine back channels
that circumvent the system's original structure.

Variable and function names might be uninformative, or even
misleading. Functions themselves may make extensive use of global
variables, as well as long lists of poorly defined parameters. The
function themselves are lengthy and convoluted, and perform several
unrelated tasks. Code is duplicated. The flow of control is hard to
understand, and difficult to follow. The programmers intent is
next to impossible to discern. The code is simply unreadable, and
borders on indecipherable. The code exhibits the unmistakable signs of
patch after patch at the hands of multiple maintainers, each of whom
barely understood the consequences of what he or she was doing. Did we
mention documentation? What documentation?

BIG BALL OF MUD might be thought
of as an anti-pattern, since our intention is to show how passivity in
the face of forces that undermine architecture can lead to a quagmire.
However, its undeniable popularity leads to the inexorable conclusion
that it is a pattern in its own right. It is certainly a pervasive,
recurring solution to the problem of producing a working system in the
context of software development. It would seem to be the path of least
resistance when one confronts the sorts of forces discussed above.
Only by understanding the logic of its appeal can we channel or
counteract the forces that lead to a BIG
BALL OF MUD.

One thing that isnt the answer is rigid, totalitarian,
top-down design. Some analysts, designers, and architects have an
exaggerated sense of their ability to get things right up-front,
before moving into implementation. This approach leads to inefficient
resources utilization, analysis paralysis, and design straightjackets
and cul-de-sacs.

Kent Beck has observed that the way to build software is to:
Make it work. Make it right. Make it fast [Beck 1997]. "Make it
work" means that we should focus on functionality up-front, and
get something running. "Make it right" means that we should
concern ourselves with how to structure the system only after
weve figured out the pieces we need to solve the problem in the
first place. "Make it fast" means that we should be
concerned about optimizing performance only after weve learned
how to solve the problem, and after weve discerned an
architecture to elegantly encompass this functionality. Once all this
has been done, one can consider how to make it cheap.

When it comes to software architecture, form follows function.
Here we mean "follows" not in the traditional sense of dictating
function. Instead, we mean that the distinct identities of the
systemís architectural elements often donít start to emerge until after
the code is working.

Domain experience is an essential ingredient in any framework
design effort. It is hard to try to follow a front-loaded, top-down
design process under the best of circumstances. Without knowing the
architectural demands of the domain, such an attempt is premature, if
not foolhardy. Often, the only way to get domain experience early in
the lifecycle is to hire someone who has worked in a domain before
from someone else.

The quality of ones tools can influence a systems
architecture. If a systems architectural goals are inadequately
communicated among members of a team, they will be harder to take into
account as the system is designed and constructed.

Finally, engineers will differ in their levels of skill and commitment
to architecture. Sadly, architecture has been undervalued for so long
that many engineers regard life with a BIG
BALL OF MUD as normal. Indeed some engineers are particularly skilled
at learning to navigate these quagmires, and guiding others through
them. Over time, this symbiosis between architecture and skills can
change the character of the organization itself, as swamp guides
become more valuable than architects. As per CONWAYS
LAW [Coplien 1995], architects depart in futility, while engineers
who have mastered the muddy details of the system they have built in
their images prevail. [Foote
& Yoder 1998a] went so far as to observe that inscrutable code
might, in fact, have a survival advantage over good code, by virtue of
being difficult to comprehend and change. This advantage can extend to
those programmers who can find their ways around such code. In a land
devoid of landmarks, such guides may become indispensable.

The incentives that drive the evolution of such systems can, at
times, operate perversely. Just as it is easier to be verbose than
concise, it is easier to build complex systems than it is to build
simple ones. Skilled programmers may be able to create complexity more
quickly than their peers, and more quickly than they can document and
explain it. Like an army outrunning its logistics train, complexity
increases until it reaches the point where such programmers can no
longer reliably cope with it.

This is akin to a phenonmenon dubbed the PeterPrinciple
of Programming by authors on the Wiki-Wiki web [Cunninghan 1999a].
Complexity increases rapidly until the it reaches a level of
complexity just beyond that with which programmers can comfortably
cope. At this point, complexity and our abilities to contain it reach
an uneasy equilibrium. The blitzkrieg bogs down into a siege. We built
the most complicated system that can possible work [Cunningham
1999b].

Such code can become a personal fiefdom, since the author care
barely understand it anymore, and no one else can come close. Once
simple repairs become all day affairs, as the code turns to mud. It
becomes increasingly difficult for management to tell how long such
repairs ought to take. Simple objectives turn into trench warfare.
Everyone becomes resigned to a turgid pace. Some even come to prefer
it, hiding in their cozy foxholes, and making their two line-per-day
repairs.

It is interesting to ask whether some of the differences in
productivity seen between hyper-productive organizations and typical
shops are due not to differences in talent, but differences in
terrain. Mud is hard to march through. The hacker in the trenches must
engage complexity in hand-to-hand combat every day. Sometimes,
complexity wins.

Status in the programmer's primate pecking order is often earned
through ritual displays of cleverness, rather than through
workman-like displays of simplicity and clarity. That which a culture
glorifies will flourish.

Yet, a case can be made that the casual, undifferentiated structure of
a BIG BALL OF MUD is one of its
secret advantages, since forces acting between two parts of the system
can be directly addressed without having to worry about undermining
the systems grander architectural aspirations. These aspirations
are modest ones at best in the typical BIG
BALL OF MUD. Indeed, a casual approach to architecture is emblematic
of the early phases of a systems evolution, as programmers,
architects and users learn their way around the domain [Foote & Opdyke
1995]. During the PROTOTYPE
and EXPANSIONARY
PHASES of a systems evolution, expedient, white-box inheritance-based
code borrowing, and a relaxed approach to encapsulation are common.
Later, as experience with the system accrues, the grain of the
architectural domain becomes discernable, and more durable black-box
components begin to emerge. In other words, its okay if the
system looks at first like a BIG
BALL OF MUD, at least until you know better.

They also can emerge as gradual maintenance and PIECEMEAL GROWTH impinges upon
the structure of a mature system. Once a system is working, a good way
to encourage its growth is to KEEP
IT WORKING. When the SHEARING
LAYERS that emerge as change drives the system's evolution run
against the existing grain of the system, its structure can be
undermined, and the result can be a BIG
BALL OF MUD.

However, these activities, which can undermine a system's structure
should be interspersed with CONSOLIDATION
PHASES [Foote & Opdyke
1995], during which opportunities to refactor the system to enhance
its structure are exploited. Proponents of Extreme Programming [Beck
2000] also emphasize continuous coding and refactoring.

[Brand 1994] observes that buildings with large spaces punctuated with
regular columns had the paradoxical effect of encouraging the
innovative reuse of space precisely because they constrained the design space.
Grandiose flights of architectural fancy werent possible, which
reduced the number of design alternatives that could be put on the
table. Sometimes FREEDOM
FROM CHOICE [Foote 1988] is what we really want.

One of mud's most effective enemies is sunshine. Subjecting
convoluted code to scrutiny can set the stage for its refactoring,
repair, and rehabilitation. Code reviews are one mechanism one can use
to expose code to daylight.

Another is the Extreme
Programming practice of pair programming [Beck 2000]. A pure pair
programming approach requires that every line of code written be added
to the system with two programmers present. One types, or "drives",
while the other "rides shotgun" and looks on. In contrast to
traditional solitary software production practices, pair programming
subjects code to immediate scrutiny, and provides a means by which
knowledge about the system is rapidly disseminated.

Indeed, reviews and pair programming provide programmers with
something their work would not otherwise have: an audience. Sunlight,
it is said is a powerful disinfectant. Pair-practices add an element
of performance to programming. An immediate audience of one's peers
provides immediate incentives to programmers to keep their code clear
and comprehensible, as well as functional.

An additional benefit of pairing is that accumulated wisdom and best
practices can be rapidly
disseminated throughout an organization through successive pairings.
This is, incidentally, the same benefit that sexual reproduction
brought to the genome.

By contrast, if no one ever looks at code, everyone is free to
think they are better than average at producing it. Programmers will,
instead, respond to those relatively perverse incentives that do
exist. Line of code metrics, design documents, and other indirect
measurements of progress and quality can become central concerns.

There are three ways to deal with BIG
BALLS OF MUD. The first is to keep the system healthy.
Conscientiously alternating periods of EXPANSION with periods
of CONSOLIDATION,
refactoring and repair can maintain, and even enhance a system's
structure as it evolves. The second is to throw the system away and
start over. The RECONSTRUCTION
pattern explores this drastic, but frequently necessary alternative.
The third is to simply surrender to entropy, and wallow in the mire.

Since the time of Roman architect Marcus
Vitruvius, [Vitruvius
20 B.C.] architects have focused on his trinity of desirables: Firmitas
(strength), Utilitas (utility), and Venustas
(beauty). A BIG BALL OF MUD
usually represents a triumph of utility over aesthetics, because
workmanship is sacrificed for functionality. Structure and durability
can be sacrificed as well, because an incomprehensible program defies
attempts at maintenance. The frenzied, feature-driven "bloatware"
phenomenon seen in many large consumer software products can be seen
as evidence of designers having allowed purely utilitarian concerns to
dominate software design.

THROWAWAY
CODE

alias

QUICK HACK

KLEENEX CODE

DISPOSABLE CODE

SCRIPTING

KILLER DEMO

PERMANENT PROTOTYPE

BOOMTOWN

vvv

A homeowner might erect a temporary storage shed or car port,
with every intention of quickly tearing it down and replacing it with
something more permanent. Such structures have a way of enduring
indefinitely. The money expected to replace them might not become
available. Or, once the new structure is constructed, the temptation
to continue to use the old one for "a while" might be hard
to resist.

Likewise, when you are prototyping a system, you are not usually
concerned with how elegant or efficient your code is. You know that
you will only use it to prove a concept. Once the prototype is done,
the code will be thrown away and written properly. As the time nears
to demonstrate the prototype, the temptation to load it with
impressive but utterly inefficient realizations of the systems
expected eventual functionality can be hard to resist. Sometimes, this
strategy can be a bit too successful. The client, rather than funding
the next phase of the project, may slate the prototype itself for
release.

You need an immediate fix
for a small problem, or a quick prototype or proof of concept.

Time, or a lack thereof, is frequently the
decisive force that drives programmers to write THROWAWAY CODE. Taking the time to
write a proper, well thought out, well documented program might take
more time that is available to solve a problem, or more time that the
problem merits. Often, the programmer will make a frantic dash to
construct a minimally functional program, while all the while
promising him or herself that a better factored, more elegant version
will follow thereafter. They may know full well that building a
reusable system will make it easier to solve similar problems in the
future, and that a more polished architecture would result in a
system that was easier to maintain and extend.

Quick-and-dirty coding is often rationalized as being a
stopgap measure. All too often, time is never found for this follow
up work. The code languishes, while the program flourishes.

THROWAWAY CODE is often written
as an alternative to reusing someone elses more complex code.
When the deadline looms, the certainty that you can produce a sloppy
program that works yourself can outweigh the unknown cost of learning
and mastering someone elses library or framework.

Programmers are usually not domain experts, especially at first. Use
cases or CRCcards [Beck &
Cunningham 1989] can help them to discover domain objects. However,
nothing beats building a prototype to help a team learn its way around
a domain.

When you build a prototype, there is always the risk that
someone will say "that's good enough, ship it". One way to minimize
the risk of a prototype being put into production is to write the
prototype in using a language or tool that you couldn't possibly use
for a production version of your product. Proponents of

Extreme Programming [Beck
2000] often construct quick, disposable prototypes called "spike
solutions". Prototypes help us learn our way around the problem space,
but should never be mistaken for good designs [Johnson & Foote 1988].

Not every program need be a palace. A simple throwaway program is like
a tent city or a mining boomtown, and often has no need for fifty year
solutions to its problems, given that it will give way to a ghost town
in five.

The production of THROWAWAY CODE
is a nearly universal practice. Any software developer, at any skill
or experience level, can be expected to have had at least occasional
first-hand experience with this approach to software development. For
example, in the patterns community, two examples of quick-and-dirty
code that have endured are the PLoP
online registration code, and the Wiki-Wiki Web
pages.

The EuroPLoP/PLoP/UP online registration code was, in effect, a
distributed web-based application that ran on four different machines
on two continents. Conference information was maintained on a machine
in St. Louis, while registration records were kept on machines in
Illinois and Germany. The system could generate web-based reports of
registration activity, and now even instantaneously maintaineed an
online attendees list. It began life in 1995 as a quick-and-dirty
collection of HTML, scavenged C demonstration code, and csh scripts.
It was undertaken largely as an experiment in web-based form
processing prior to PLoP 95, and, like so many things on the
Web, succeeded considerably beyond the expectations of its authors.
Today, it is still essentially the same collection of HTML, scavenged
C demonstration code, and csh scripts. As such, it showcases how
quick-and-dirty code can, when successful, take on a life of its own.

The original C code and scripts probably contained fewer than
three dozen original lines of code. Many lines were cut-and-paste jobs
that differed only in the specific text they generate, or fields that
they check.

Heres an example of one of the scripts that generates the
attendance report:

This script is slow and inefficient, particularly as the number of
registrations increases, but not least among its virtues is the fact
that it works. Were the number of attendees to exceed more than
around one hundred, this script would start to perform so badly as to
be unusable. However, since hundreds of attendees would exceed the
physical capacity of the conference site, we knew the number of
registrations would have been limited long before the performance of
this script became a significant problem. So while this approach is,
in general, a lousy way to address this problem, it is perfectly
satisfactory within the confines of the particular purpose for which
the script has ever actually been used. Such practical constraints are
typical of THROWAWAY CODE, and
are more often than not undocumented. For that matter, everything
about THROWAWAY CODE is more
often than not undocumented. When documentation exists, it is
frequently not current, and often not accurate.

The Wiki-Web code
at www.c2.com also started as a CGI
experiment undertaken by Ward Cunningham also succeeded beyond the
authors expectations. The name "wiki" is one of
Wards personal jokes, having been taken from a Hawaiian word for
"quick" that the author had seen on an airport van on a
vacation in Hawaii. Ward has subsequently used the name for a number
of quick-and-dirty projects. The Wiki Web is unusual in that any
visitor may change anything that anyone else has written
indiscriminately. This would seem like a recipe for vandalism, but in
practice, it has worked out well. In light of the systems
success, the author has subsequently undertaken additional work to
polish it up, but the same quick-and-dirty Perl CGI core remains at
the heart of the system.

Both systems might be thought of as being on the verge of graduating
from little balls of mud to BIG
BALLS OF MUD. The registration systems C code metastasized from one of
the NCSA HTTPD server demos, and still contains zombie code that
testifies to this heritage. At each step, KEEPING IT WORKING is a premiere
consideration in deciding whether to extend or enhance the system.
Both systems might be good candidates for RECONSTRUCTION, were the
resources, interest, and audience present to justify such an
undertaking. In the mean time, these systems, which are still
sufficiently well suited to the particular tasks for which they were
built, remain in service. Keeping them on the air takes far less
energy than rewriting them. They continue to evolve, in a PIECEMEAL fashion, a little at a
time.

You can ameloriate the architectural erosion that can be caused
by quick-and-dirty code by isolating it from other parts of your
system, in its own objects, packages, or modules. To the extent that
such code can be quarantined, its ability to affect the integrity of
healthy parts of a system is reduced.

Once it becomes evident that a purportedly disposable artifact is
going to be around for a while, one can turn one's attention to
improving its structure, either through an iterative process of PIECEMEAL GROWTH, or via a fresh
draft, as discussed in the RECONSTRUCTION
pattern.

From boomtown to ghost
town: The mining town of Rhyolite, in Death Valley,
was briefly the third largest city in Nevada. Then the ore
ran out.

PIECEMEAL GROWTH

alias

URBAN SPRAWL

ITERATIVE-INCREMENTAL
DEVELOPMENT

The Russian Mir ("Peace") Space Station
Complex was designed
for reconfiguration and modular
growth. The Core module was launched in 1986, and the Kvant
("Quantum") and Kvant-2 modules joined the complex in 1987 and 1989.
The Kristall ("Crystal") module was added in 1990. The Spektr
("Spectrum") and shuttle Docking modules were added in 1995, the
latter surely a development
not anticipated in 1986. The stations final module, Priroda
("Nature"), was launched in 1996. The common core and independent
maneuvering capabilities of several of the modules have allowed the
complex to be rearranged
several times as it has grown.

Urban planning has an uneven
history of success. For instance, Washington D.C. was laid out
according to a master plan
designed by the French architect LEnfant. The
capitals of
Brazil (Brasilia)
and Nigeria (Abuja)
started as paper cities as well. Other cities, such as Houston,
have grown without any overarching plan to guide them. Each approach
has its problems. For instance, the radial street plans in
LEnftants master plan become awkward past a certain
distance from the center. The lack of any plan at all, on the other
hand, leads to a patchwork of residential, commercial, and industrial
areas that is dictated by the capricious interaction of local forces
such as land ownership, capital, and zoning. Since concerns such as
recreation, shopping close to homes, and noise and pollution away from
homes are not brought directly into the mix, they are not adequately
addressed.

Most cities are more like Houston than Abuja. They may begin as
settlements, subdivisions, docks, or railway stops. Maybe people were
drawn by gold, or lumber, access to transportation, or empty land. As
time goes on, certain settlements achieve a critical mass, and a
positive feedback cycle ensues. The citys success draws
tradesmen, merchants, doctors, and clergymen. The growing population
is able to support infrastructure, governmental institutions, and
police protection. These, in turn, draw more people. Different
sections of town develop distinct identities. With few exceptions,
(Salt Lake City comes to mind) the founders of these settlements never
stopped to think that they were founding major cities. Their ambitions
were usually more modest, and immediate.

vvv

It has become fashionable over the last several years to take
pot shots at the "traditional" waterfall process model. It may seem to
the reader that attacking it is tantamount to flogging a dead horse.
However, if it be a dead horse, it is a tenacious one. While the
approach itself is seen by many as having been long since discredited,
it has spawned a legacy of rigid, top-down, front-loaded processes and
methodologies that endure, in various guises, to this day. We can do
worse that examine the forces that led to its original development.

In the days before waterfall development, programming pioneers
employed a simple, casual, relatively undisciplined "code-and-fix"
approach to software development. Given the primitive nature of the
problems of the day, this approach was frequently effective. However,
the result of this lack of discipline was, all too often, a BIG BALL OF MUD.

The waterfall approach arose in response to this muddy morass.
While the code-and-fix approach might have been suitable for small
jobs, it did not scale well. As software became more complex, it would
not do to simply gather a room full of programmers together and tell
them to go forth and code. Larger projects demanded better planning
and coordination. Why, it was asked, can't software be engineered like
cars and bridges, with a careful analysis of the problem, and a
detailed up-front design prior to implementation? Indeed, an
examination of software development costs showed that problems were
many times more expensive to fix during maintenance than during
design. Surely it was best to mobilize resources and talent up-front,
so as to avoid maintenance expenses down the road. It's surely wiser
to route the plumbing correctly now, before the walls are up, than to
tear holes in them later. Measure twice, cut once.

One of the reasons that the waterfall approach was able to
flourish a generation ago was that computers and business requirements
changed at a more leisurely pace. Hardware was very expensive, often
dwarfing the salaries of the programmers hired to tend it. User
interfaces were primitive by today's standards. You could have any
user interface you wanted, as long as it was an alphanumeric "green
screen". Another reason for the popularity of the waterfall approach
was that it exhibited a comfortable similarity to practices in more
mature engineering and manufacturing disciplines.

Today's designers are confronted with a broad onslaught of
changing requirements. It arises in part from the rapid growth of
technology itself, and partially from rapid changes in the business
climate (some of which is driven by technology). Customers are used to
more sophisticated software these days, and demand more choice and
flexibility. Products that were once built from the ground up by
in-house programmers must now be integrated with third-party code and
applications. User interfaces are complex, both externally and
internally. Indeed, we often dedicate an entire tier of our system to
their care and feeding. Change threatens to outpace our ability to
cope with it.

Master plans are often
rigid, misguided and out of date. Users needs change with
time.

Change: The fundamental problem with top-down design is that
real world requirement are inevitably moving targets. You can't simply
aspire to solve the problem at hand once and for all, because, by the
time you're done, the problem will have changed out from underneath
you. You can't simply do what the customer wants, for quite often,
they don't know what they want. You can't simply plan, you have to
plan to be able to adapt. If you can't fully anticipate what is going
to happen, you must be prepared to be nimble.

Aesthetics: The goal of up-front design is to be able to
discern and specify the significant architectural elements of a system
before ground is broken for it. A superior design, given this mindset,
is one that elegantly and completely specifies the system's structure
before a single line of code has been written. Mismatches between
these blueprints and reality are considered aberrations, and are
treated as mistakes on the part of the designer. A better design would
have anticipated these oversights. In the presence of volatile
requirements, aspirations towards such design perfection are as vain
as the desire for a hole-in-one on every hole.

To avoid such embarrassment, the designer may attempt to cover
him or herself by specifying a more complicated, and more general
solution to certain problems, secure in the knowledge that others will
bear the burden of constructing these artifacts. When such predictions
about where complexity is needed are correct, they can indeed be a
source of power and satisfaction. This is part of their allure of
Venustas. However, sometime the anticipated contingencies never arise,
and the designer and implementers wind up having wasted effort solving
a problem that no one has ever actually had. Other times, not only is
the anticipated problem never encountered, its solution introduces
complexity in a part of the system that turns out to need to evolve in
another direction. In such cases, speculative complexity can be an
unnecessary obstacle to subsequent adaptation. It is ironic that the
impulse towards elegance can be an unintended source of complexity and
clutter instead.

In its most virulent form, the desire to anticipate and head off
change can lead to "analysis paralysis", as the thickening web of
imagined contingencies grows to the point where the design space seems
irreconcilably constrained.

Therefore, incrementally
address forces that encourage change and growth. Allow
opportunities for growth to be exploited locally, as they
occur. Refactor unrelentingly.

Successful software attracts a wider audience, which can, in turn,
place a broader range of requirements on it. These new requirements
can run against the grain of the original design. Nonetheless, they
can frequently be addressed, but at the cost of cutting across the
grain of existing architectural assumptions. [Foote 1988] called this
architectural erosion midlife generality loss.

When designers are faced with a choice between building something
elegant from the ground up, or undermining the architecture of the
existing system to quickly address a problem, architecture usually
loses. Indeed, this is a natural phase in a systems evolution [Foote & Opdyke 1995]. This
might be thought of as messy kitchen phase, during which pieces
of the system are scattered across the counter, awaiting an eventual
cleanup. The danger is that the clean up is never done. With real
kitchens, the board of health will eventually intervene. With
software, alas, there is seldom any corresponding agency to police
such squalor. Uncontrolled growth can ultimately be a malignant force.
The result of neglecting to contain it can be a BIG BALL OF MUD.

In How Buildings Learn, Brand [Brand 1994] observed that what
he called High Road architecture often resulted in buildings
that were expensive and difficult to change, while vernacular, Low
Road buildings like bungalows and warehouses were, paradoxically,
much more adaptable. Brand noted that Function melts form, and
low road buildings are more amenable to such change. Similarly, with
software, you may be reluctant to desecrate another programmers
cathedral. Expedient changes to a low road system that exhibits no
discernable architectural pretensions to begin with are easier to
rationalize.

Large-lump development is based on the idea of replacement.
Piecemeal Growth is based on the idea of repair.
Large-lump development is based on the fallacy that it is possible to
build perfect buildings. Piecemeal growth is based on the healthier
and more realistic view that mistakes are inevitable. Unless
money is available for repairing these mistakes, every building, once
built, is condemned to be, to some extent unworkable.
Piecemeal growth is based on the assumption that adaptation between
buildings and their users is necessarily a slow and continuous
business which cannot, under any circumstances, be achieve in a
single leap.

Alexander has noted that our mortgage and capital expenditure
policies make large sums of money available up front, but do nothing
to provide resources for maintenance, improvement, and evolution
[Brand 1994][Alexander 1988]. In the software world, we deploy our
most skilled, experienced people early in the lifecycle. Later on,
maintenance is relegated to junior staff, when resources can be
scarce. The so-called maintenance phase is the part of the lifecycle
in which the price of the fiction of master planning is really paid.
It is maintenance programmers who are called upon to bear the burden
of coping with the ever widening divergence between fixed designs and
a continuously changing world. If the hypothesis that architectural
insight emerges late in the lifecycle is correct, then this practice
should be reconsidered.

Brand went on to observe Maintenance is learning.
He distinguishes three levels of learning in the context of systems.
This first is habit, where a system dutifully serves its function
within the parameters for which it was designed. The second level
comes into play when the system must adapt to change. Here, it usually
must be modified, and its capacity to sustain such modification
determines its degree of adaptability. The third level is the
most interesting: learning to learn. With buildings, adding a
raised floor is an example. Having had to sustain a major upheaval,
the system adapts so that subsequent adaptations will be much less
painful.

PIECEMEAL GROWTH can be
undertaken in an opportunistic fashion, starting with the existing,
living, breathing system, and working outward, a step at a time, in
such a way as to not undermine the systems viability. You
enhance the program as you use it. Broad advances on all fronts are
avoided. Instead, change is broken down into small, manageable chunks.

One of the most striking things about PIECEMEAL GROWTH is the role
played by Feedback. Herbert Simon [Simon 1969] has observed
that few of the adaptive systems that have been forged by evolution or
shaped by man depend on prediction as their main means of coping with
the future. He notes that two complementary mechanisms, homeostasis,
and retrospective feedback, are often far more effective. Homeostasis
insulates the system from short-range fluctuations in its environment,
while feedback mechanisms respond to long-term discrepancies between a
system's actual and desired behavior, and adjust it accordingly.
Alexander [Alexander 1964] has written extensively of the roles that
homeostasis and feedback play in adaptation as well.

If you can adapt quickly to change, predicting it becomes far less
crucial. Hindsight, as Brand observes [Brand 1994] is better than
foresight. Such rapid adaptation is the basis of one of the mantras of
Extreme Programming [Beck
2000]: You're not going to need it.

Proponents of XP (as it is called) say to pretend you are not a
smart as you think you are, and wait until this clever idea of yours
is actually required before you take the time to bring it into being.
In the cases where you were right, hey, you saw it coming, and you
know what to do. In the cases where you were wrong, you won't have
wasted any effort solving a problem you've never had when the design
heads in an unanticipated direction instead.

Extreme Programming relies
heavily on feedback to keep requirements in sync with code, by
emphasizing short (three week) iterations, and extensive, continuous
consultation with users regarding design and development priorities
throughout the development process. Extreme Programmers do not engage
in extensive up-front planning. Instead, they produce working code as
quickly as possible, and steer these prototypes towards what the users
are looking for based on feedback.

Feedback also plays a role in determining coding assignments.
Coders who miss a deadline are assigned a different task during the
next iteration, regardless of how close they may have been to
completing the task. This form of feedback resembles the stern justice
meted out by the jungle to the fruit of uncompetitive pairings.

Extreme Programming also
emphasizes testing as an integral part of the development process.
Tests are developed, ideally, before the code itself. Code is
continuously tested as it is developed.

There is a "back-to-the-future" quality to Extreme Programming. In
many respects, it resembles the blind Code and Fix approach.
The thing that distinguishes it is the central role played by feedback
in driving the system's evolution. This evolution is abetted, in turn,
by modern object-oriented languages and powerful refactoring tools.

Proponents of extreme programming portray it as placing minimal
emphasis on planning and up-front design. They rely instead on
feedback and continuous integration. We believe that a certain amount
of up-front planning and design is not only important, but inevitable.
No one really goes into any project blindly. The groundwork must be
laid, the infrastructure must be decided upon, tools must be selected,
and a general direction must be set. A focus on a shared architectural
vision and strategy should be established early.

A broad consensus that objects emerge from an iterative
incremental evolutionary process has formed in the object-oriented
community over the last decade. See for instance [Booch 1994]. The SOFTWARE
TECTONICS pattern [Foote & Yoder 1996] examines how systems can
incrementally cope with change.

The biggest risk associated with PIECEMEAL
GROWTH is that it will gradually erode the overall structure of the
system, and inexorably turn it into a BIG
BALL OF MUD. A strategy of KEEPING
IT WORKING goes hand in hand with PIECEMEAL
GROWTH. Both patterns emphasize acute, local concerns at the expense
of chronic, architectural ones.

To counteract these forces, a permanent commitment to CONSOLIDATION and
refactoring must be
made. It is through such a process that local and global forces are
reconciled over time. This lifecyle perspective has been dubbed the fractal model [Foote & Opdyke 1995]. To
quote Alexander [Brand 1994][Alexander 1988]:

An organic process of growth and repair must create a gradual
sequence of changes, and these changes must be distributed evenly
across all levels of scale. [In developing a college campus] there
must be as much attention to the repair of detailsrooms, wings
of buildings, windows, pathsas to the creation of brand new
buildings. Only then can the environment be balanced both as a whole,
and in its parts, at every moment in its history.

KEEP IT WORKING

alias

VITALITY

BABY STEPS

DAILY BUILD

FIRST, DO NO HARM

Probably the greatest factor
that keeps us moving forward is that we use the system all the time,
and we keep trying to do new things with it. It is this
"living-with" which drives us to root out failures, to
clean up inconsistencies, and which inspires our occasional
innovation.

Daniel H. H. Ingalls [Ingalls
1983]

Once a city establishes its infrastructure, it is imperative
that it be kept working. For example, if the sewers break, and
arent quickly repaired, the consequences can escalate from
merely unpleasant to genuinely life threatening. People come to expect
that they can rely on their public utilities being available 24 hours
per day. They (rightfully) expect to be able to demand that an outage
be treated as an emergency.

vvv

Software can be like this. Often a business becomes dependent
upon the data driving it. Businesses have become critically dependent
on their software and computing infrastructures. There are numerous
mission critical systems that must be on-the-air twenty-four hours a
day/seven days per week. If these systems go down, inventories can not
be checked, employees can not be paid, aircraft cannot be routed, and
so on.

There may be times where taking a system down for a major overhaul can
be justified, but usually, doing so is fraught with peril. However,
once the system is brought back up, it is difficult to tell which from
among a large collection of modifications might have caused a new
problem. Every change is suspect. This is why deferring such
integration is a recipe for misery. Capers Jones [Jones 1999] reported
that the chance that a significant change might contain a new error--a
phenomenon he ominously referred to as a Bad Fix Injection--
was about 7% in the United States. This may strike some readers as a
low figure. Still, it's easy to see that compounding this possibility
can lead to a situation where multiple upgrades are increasing likely
to break a system.

Maintenance needs have accumulated, but
an overhaul is unwise, since you might break the system.

Workmanship: Architects who live in the house they are building
have an obvious incentive to insure that things are done properly,
since they will directly reap the consequences when they do not. The
idea of the architect-builder is a central theme of Alexander's work.
Who better to resolve the forces impinging upon each design issue as
it arises as the person who is going to have to live with these
decisions? The architect-builder will be the direct beneficiary of his
or her own workmanship and care. Mistakes and shortcuts will merely
foul his or her own nest.

Dependability: These days, people rely on our software
artifacts for their very livelihoods, and even, at time, for their
very safety. It is imperative that ill-advise changes to elements of a
system do not drag the entire system down. Modern software systems are
intricate, elaborate webs of interdependent elements. When an
essential element is broken, everyone who depends on it will be
affected. Deadlines can be missed, and tempers can flare. This problem
is particularly acute in BIG BALLS
OF MUD, since a single failure can bring the entire system down like
a house of cards.

Therefore, do what it
takes to maintain the software and keep it going. Keep it working.

When you are living in the system youíre building, you have an
acute incentive not to break anything. A plumbing outage will be a
direct inconvenience, and hence you have a powerful reason to keep it
brief. You are, at times, working with live wires, and must exhibit
particular care. A major benefit of working with a live system is that
feedback is direct, and nearly immediate.

One of the strengths of this strategy is that modifications that break
the system are rejected immediately. There are always a large number
of paths forward from any point in a systems evolution, and most
of them lead nowhere. By immediately selecting only those that do not
undermine the systems viability, obvious dead-ends are avoided.

Of course, this sort of reactive approach, that of kicking the
nearest, meanest woolf from your door, is not necessarily globally
optimal. Yet, by eliminating obvious wrong turns, only more
insidiously incorrect paths remain. While these are always harder to
identify and correct, they are, fortunately less numerous than those
cases where the best immediate choice is also the best overall choice
as well.

It may seem that this approach only accommodates minor
modifications. This is not necessarily so. Large new subsystems might
be constructed off to the side, perhaps by separate teams, and
integrated with the running system in such a way as to minimize
distruption.

Design space might be thought of as a vast, dark, largely
unexplored forest. Useful potential paths through it might be thought
of as encompassing working programs. The space off to the sides of
these paths is much larger realm of non-working programs. From any
given point, a few small steps in most directions take you from a
working to a non-working program. From time to time, there are forks
in the path, indicating a choice among working alternatives. In
unexplored territory, the prudent strategy is never to stray too far
from the path. Now, if one has a map, a shortcut through the trekless
thicket that might save miles may be evident. Of course, pioneers, by
definition, dont have maps. By taking small steps in any
direction, they know that it is never more than a few steps back to a
working system.

Some years ago, Harlan Mills
proposed that any software system should be grown by incremental
development. That is, the system first be made to run, even though it
does nothing useful except call the proper set of dummy subprograms.
Then, bit by bit, it is fleshed out, with the subprograms in turn
being developed into actions or calls to empty stubs in the level
below.

Nothing in the past decade has
so radically changed my own practice, and its effectiveness.

One always has, at every stage,
in the process, a working system. I find that teams can grow
much more complex entities in four months than they can build.

-- From "No Silver
Bullet" [Brooks 1995]

Microsoft mandates that a DAILY BUILD of each product be
performed at the end of each working day. Nortel adheres to the
slightly less demanding requirement that a working build be generated
at the end of each week [Brooks 1995][Cusumano & Shelby 1995].
Indeed, this approach, and keeping the last working version around,
are nearly universal practices among successful maintenance
programmers.

Another vital factor in ensuring a system's continued vitality
is a commitment to rigorous testing [Marick 1995][Bach 1994]. It's
hard to keep a system working if you don't have a way of making sure
it works. Testing is one of pillars of Extreme Programming. XP
practices call for the development of unit tests before a single line
of code is written.

vvv

Always beginning with a working system helps to encourage PIECEMEAL GROWTH. Refactoring is
the primary means by which programmers maintain order from inside the
systems in which they are working. The goal of refactoring is to leave
a system working as well after a refactoring as it was before the
refactoring. Aggressive unit and integration testing can help to
guarantee that this goal is met.

SHEARING LAYERS

Hummingbirds and flowers are
quick, redwood trees are slow, and whole redwood forests are even
slower. Most interaction is within the same pace level--hummingbirds
and flowers pay attention to each other, oblivious to redwoods, who
are oblivious to them.

R. V. O'Neill , A
Hierarchical Concept of Ecosystems

The notion of SHEARING LAYERS is
one of the centerpieces of Brand's How Buildings Learn [Brand
1994]. Brand, in turn synthesized his ideas from a variety of sources,
including British designer Frank Duffy, and ecologist R. V. O'Neill.

Brand quotes Duffy as saying: "Our basic argument is that there
isn't any such thing as a building. A building properly conceived is
several layers of longevity of built components".

Brand distilled Duffy's proposed layers into these six: Site,
Structure, Skin, Services, Space Plan, and Stuff. Site is geographical
setting. Structure is the load bearing elements, such as the
foundation and skeleton. Skin is the exterior surface, such as siding
and windows. Services are the circulatory and nervous systems of a
building, such as its heating plant, wiring, and plumbing. The Space
Plan includes walls, flooring, and ceilings. Stuff includes lamps,
chairs, appliances, bulletin boards, and paintings.

These layers change at different rates. Site, they say, is
eternal. Structure may last from 30 to 300 years. Skin lasts for
around 20 years, as it responds to the elements, and to the whims of
fashion. Services succumb to wear and technical obsolescence more
quickly, in 7 to 15 years. Commercial Space Plans may turn over every
3 years. Stuff, is, of course, subject to unrelenting flux [Brand
1994].

vvv

Software systems cannot stand still. Software is often called upon to
bear the brunt of changing requirements, because, being as that it is
made of bits, it can change.

Different artifacts change
at different rates.

Adaptability: A system that can cope readily with a wide range
of requirements, will, all other things being equal, have an advantage
over one that cannot. Such a system can allow unexpected requirements
to be met with little or no reengineering, and allow its more skilled
customers to rapidly address novel challenges.

Stability: Systems succeed by doing what they were designed to
do as well as they can do it. They earn their niches, by bettering
their competition along one or more dimensions such as cost, quality,
features, and performance. See [Foote
& Roberts 1998] for a discussion of the occasionally fickle nature of
such completion. Once they have found their niche, for whatever
reason, it is essential that short term concerns not be allowed to
wash away the elements of the system that account for their mastery of
their niche. Such victories are inevitably hard won, and fruits of
such victories should not be squandered. Those parts of the system
that do what the system does well must be protected from fads, whims,
and other such spasms of poor judgement.

Adaptability and Stability are forces that are in
constant tension. On one hand, systems must be able to confront
novelty without blinking. On the other, they should not squander their
patrimony on spur of the moment misadventures.

Therefore, factor your
system so that artifacts that change at similar rates are together.

Most interactions in a system tend to be within layers, or between
adjacent layers. Individual layers tend to be about things that change
at similar rates. Things that change at different rates diverge.
Differential rates of change encourage layers to emerge. Brand notes
as well that occupational specialties emerge along with these layers.
The rate at which things change shapes our organizations as well. For
instance, decorators and painters concern themselves with interiors,
while architects dwell on site and skin. We expect to see things that
evolve at different rates emerge as distinct concerns. This is "separate
that which changes from that which doesn't" [Roberts
& Johnson 1998] writ large.

Can we identify such layers in software?

Well, at the bottom, there are data. Things that change most
quickly migrate into the data, since this is the aspect of software
that is most amenable to change. Data, in turn, interact with users
themselves, who produce and consume them.

Code changes more slowly than data, and is the realm of
programmers, analysts and designers. In object-oriented languages,
things that will change quickly are cast as black-box polymorphic
components. Elements that will change less often may employ white-box
inheritance.

The abstract classes and components that constitute an
object-oriented framework change more slowly than the applications
that are built from them. Indeed, their role is to distill what is
common, and enduring, from among the applications that seeded the
framework.

As frameworks evolve, certain abstractions make their ways from
individual applications into the frameworks and libraries that
constitute the system's infrastructure [Foote 1988]. Not all elements will
make this journey. Not all should. Those that do are among the most
valuable legacies of the projects that spawn them. Objects help
shearing layers to emerge, because they provide places where more
fine-grained chunks of code and behavior that belong together can
coalesce.

The Smalltalk programming language is built from a set of
objects that have proven themselves to be of particular value to
programmers. Languages change more slowly than frameworks. They are
the purview of scholars and standards committees. One of the
traditional functions of such bodies is to ensure that languages
evolve at a suitably deliberate pace.

Artifacts that evolve quickly provide a system with dynamism and
flexibility. They allow a system to be fast on its feet in the face of
change.

Slowly evolving objects are bulwarks against change. They embody
the wisdom that the system has accrued in its prior interactions with
its environment. Like tenure, tradition, big corporations, and
conservative politics, they maintain what has worked. They worked
once, so they are kept around. They had a good idea once, so maybe
they are a better than even bet to have another one.

Wide acceptance and deployment causes resistance to change. If
changing something will break a lot of code, there is considerable
incentive not to change it. For example, schema reorganization in
large enterprise databases can be an expensive and time-consuming
process. Database designers and administrators learn to resist change
for this reason. Separate job descriptions, and separate hardware,
together with distinct tiers, help to make these tiers distinct.

The phenomenon whereby distinct concerns emerge as distinct
layers and tiers can be seen as well with graphical user interfaces.

Part of the impetus behind using METADATA [Foote & Yoder 1998b] is the
observation that pushing complexity and power into the data pushes
that same power (and complexity) out of the realm of the programmer
and into the realm of users themselves. Metadata are often used to
model static facilities such as classes and schemas, in order to allow
them to change dynamically. The effect is analogous to that seen with
modular office furniture, which allows office workers to easily,
quickly, and cheaply move partitions without having to enlist
architects and contractors in the effort.

Over time, our frameworks, abstract classes, and components come
to embody what we've learned about the structure of the domains for
which they are built. More enduring insights gravitate towards the
primary structural elements of these systems. Things which find
themselves in flux are spun out into the data, where users can
interact with them. Software evolution becomes like a centrifuge spun
by change. The layers that result, over time, can come to a much truer
accommodation with the forces that shaped them than any top-down
agenda could have devised.

Things that are good have a
certain kind of structure. You canít get that structure except
dynamically. Period. In nature youíve got continuous
very-small-feedback-loop adaptation going on, which is why things get
to be harmonious. Thatís why they have the qualities we value. If it
wasnít for the time dimension, it wouldnít happen. Yet here we are
playing the major role creating the world, and we havenít figured
this out. That is a very serious matter.

One of the most spectacular
examples of sweeping a problem under the rug is the
concrete sarcophagus that Soviet engineers constructed to put a
10,000 year lid on the infamous reactor
number four at Chernobyl,
in what is now Ukraine.

If you cant make a mess go away, at least you can hide it.
Urban renewal can begin by painting murals over graffiti and putting
fences around abandoned property. Children often learn that a single
heap in the closet is better than a scattered mess in the middle of
the floor.

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There are reasons, other than aesthetic concerns, professional
pride, and guilt for trying to clean up messy code. A deadline may be
nearing, and a colleague may want to call a chunk of your code, if you
could only come up with an interface through which it could be called.
If you dont come up with an easy to understand interface,
theyll just use someone elses (perhaps inferior) code. You
might be cowering during a code-review, as your peers trudge through a
particularly undistinguished example of your work. You know that there
are good ideas buried in there, but that if you dont start to
make them more evident, they may be lost.

There is a limit to how much chaos an individual can tolerate before
being overwhelmed. At first glance, a BIG
BALL OF MUD can inspire terror and despair in the hearts of those who
would try to tame it. The first step on the road to architectural
integrity can be to identify the disordered parts of the system, and
isolate them from the rest of it. Once the problem areas are
identified and hemmed in, they can be gentrified using a divide and
conquer strategy.

Overgrown, tangled,
haphazard spaghetti code is hard to comprehend, repair, or extend,
and tends to grow even worse if it is not somehow brought under
control.

Comprehensibility: It should go without saying that
comprehensible, attractive, well-engineered code will be easier to
maintain and extend than complicated, convoluted code. However, it
takes Time and money to overhaul sloppy code. Still, the Cost
of allowing it to fester and continue to decline should not be
underestimated.

Morale: Indeed, the price of life with a BIG BALL OF MUD goes beyond the
bottom line. Life in the muddy trenches can be a dispiriting fate.
Making even minor modifications can lead to maintenance marathons.
Programmers become timid, afraid that tugging at a loose thread may
have unpredictable consequences. After a while, the myriad threads that
couple every part of the system to every other come to tie the
programmer down as surely as Gulliver among the
Lilliputians
[Swift
1726]. Talent may desert the project in the face of such bondage.

It should go without saying that comprehensible, attractive,
well-engineered code will be easier to maintain and extend than
complicated, convoluted code. However, it takes time and money to
overhaul sloppy code. Still, the cost of allowing it to fester and
continue to decline should not be underestimated.

Therefore, if you
cant easily make a mess go away, at least cordon it off. This
restricts the disorder to a fixed area, keeps it out of sight, and
can set the stage for additional refactoring.

By getting the dirt into a single pile beneath the carpet, you
at least know where it is, and can move it around. Youve still
got a pile of dirt on your hands, but it is localized, and your guests
cant see it. As the engineers who entombed reactor number four
at Chernobly demonstrated, sometimes you've got to get a lid on a
problem before you can get serious about cleaning things up. Once the
problem area is contained, you can decontaminate at a more leisurely
pace.

To begin to get a handle on spaghetti code, find those sections
of it that seem less tightly coupled, and start to draw architectural
boundaries there. Separate the global information into distinct data
structures, and enforce communication between these enclaves using
well-defined interfaces. Such steps can be the first ones on the road
to re-establishing the systems conceptual integrity, and
discerning nascent architectural landmarks.

Putting a fresh interface around a run down region of the system can
be the first step on the way architectural rehabilitation. This is a
long row to hoe, however. Distilling meaningful abstractions from a BIG BALL OF MUD is a difficult and
demand task. It requires skill, insight, and persistence. At times, RECONSTRUCTION may seem like the
less painful course. Still, it is not like unscrambling an egg. As
with rehabilitation in the real world, restoring a system to
architectural health requires resources, as well as a sustained
commitment on the part of the people who live there.

The UIMX user interface builder for Unix and Motif, and the
various Smalltalk GUI builders both provide a means for programmers to
cordon off complexity in this fashion.

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One frequently constructs a FA«ADE
[Gamma et. al. 1995] to put a congenial
"pretty face" on the unpleasantness that is SWEPT UNDER THE RUG. Once
these messy chunks of code have been quarantined, you can expose their
functionality using INTENTION REVEALING SELECTORS [Beck 1997].

This paper also examines how complexity can be hidden using suitable
defaults (WORKS
OUT OF THE BOX and PROGRAMMING-BY-DIFFERRENCE),
and interfaces that gradually reveal additional capabilities as the
client grows more sophisticated.

RECONSTRUCTION

alias

TOTAL REWRITE

DEMOLITION

THROWAWAY THE FIRST ONE

START OVER

Atlantas Fulton County Stadium was built in 1966 to serve
as the home of baseballs Atlanta Braves, and footballs
Atlanta Falcons. In August of 1997, the stadium was demolished. Two
factors contributed to its relatively rapid obsolescence. One was that
the architecture of the original stadium was incapable of
accommodating the addition of the "sky-box" suites that the
spreadsheets of 90s sporting economics demanded. No conceivable
retrofit could accommodate this requirement. Addressing it meant
starting over, from the ground up. The second was that the
stadiums attempt to provide a cheap, general solution to the
problem of providing a forum for both baseball and football audiences
compromised the needs of both. In only thirty-one years, the balance
among these forces had shifted decidedly. The facility is being
replaced by two new single-purpose stadia.

Might there be lessons for us about unexpected requirements and
designing general components here?

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Plan to Throw One Away
(You Will Anyway) -- Brooks

Extreme Programming [Beck 2000] had its genesis in the Chrysler
Comprehensive Compensation project (C3). It began with a cry for help
from a foundering project, and a decision to discard a year and a
half's worth of work. The process they put in place after they started
anew laid the foundation for XP, and the author's credit these
approaches for the subsequent success of the C3 effort. However, less
emphasis is given to value of the experience the team might have
salvaged from their initial, unsuccessful draft. Could this first
draft have been the unsung hero of this tale?

Your code has declined to
the point where it is beyond repair, or even comprehension.

Obsolescence: Of course, one reason to abandon a system is that
it is in fact technically or economically obsolete. These are distinct
situations. A system that is no longer state-of-the-art may still sell
well, while a technically superior system may be overwhelmed by a more
popular competitor for non-technical reasons.

In the realm of concrete and steel, blight is the symptom, and a
withdrawal of capital is the cause. Of course, once this process
begins, it can feed on itself. On the other hand, given a steady
infusion of resources, buildings can last indefinitely. It's not
merely entropy, but an unwillingness to counteract it, that allows
buildings to decline. In Europe, neighborhoods have flourished for
hundreds of years. They have avoided the boom/bust cycles that
characterize some New World cities.

Change: Even though software is a highly malleable medium, like
Fulton County Stadium, new demands can, at times, cut across a
systems architectural assumptions in such a ways as to make
accommodating them next to impossible. In such cases, a total rewrite
might be the only answer.

Cost: Writing-off a system can be traumatic, both to those who
have worked on it, and to those who have paid for it. Software is
often treated as an asset by accountants, and can be an expensive
asset at that. Rewriting a system, of course, does not discard its
conceptual design, or its staffs experience. If it is truly the
case that the value of these assets is in the design experience they
embody, then accounting practices must recognize this.

Organization: Rebuilding a system from scratch is a
high-profile undertaking, that will demand considerable time and
resources, which, in turn, will make high-level management support
essential.

Therefore, throw it away
and start over.

Sometimes its just easier to throw a system away, and
start over. Examples abound. Our shelves are littered with the
discarded carcasses of obsolete software and its documentation.
Starting over can be seen as a defeat at the hands of the old code, or
a victory over it.

One reason to start over might be that the previous system was
written by people who are long gone. Doing a rewrite provides new
personnel with a way to reestablish contact between the architecture
and the implementation. Sometimes the only way to understand a system
it is to write it yourself. Doing a fresh draft is a way to overcome
neglect. Issues are revisited. A fresh draft adds vigor. You draw back
to leap. The quagmire vanishes. The swamp is drained.

Another motivation for building a new system might be that you
feel that you've got the experience you need to do the job properly.
One way to have gotten this experience is to have participated at some
level in the unsuccessful development of a previous version of the
system.

Of course, the new system is not designed in a vacuum.
Brooks famous tar pit is excavated, and the fossils are
examined, to see what they can tell the living. It is essential that a
thorough post-mortem review be done of the old system, to see what it
did well, and why it failed. Bad code can bog down a good design. A
good design can isolate and contain bad code.

When a system becomes a BIG BALL
OF MUD, its relative incomprehensibility may hasten its demise, by
making it difficult for it to adapt. It can persist, since it resists
change, but cannot evolve, for the same reason. Instead, its
inscrutability, even when it is to its s hort-term benefit, sows the
seeds of its ultimate demise.

If this makes muddiness a frequently terminal condition, is this
really a bad thing? Or is it a blessing that these sclerotic systems
yield the stage to more agile successors? Certainly, the departure of
these ramshackle relics can be a cause for celebration as well as
sadness.

Discarding a system dispenses with its implementation, and
leaves only its conceptual design behind. Only the patterns that
underlie the system remain, grinning like a Cheshire cat. It is their
spirits that help to shape the next implementation. With luck, these
architectural insights will be reincarnated as genuine reusable
artifacts in the new system, such as abstract classes and frameworks.
It is by finding these architectural nuggets that the promise of
objects and reuse can finally be fulfilled.

There are alternatives to throwning your system away and starting
over. One is to embark on a regimen of incremental refactoring, to
glean architectural elements and discernable abstractions from the
mire. Indeed, you can begin by looking for coarse fissures along which
to separate parts of the system, as was suggested in SWEEPING IT UNDER THE RUG.
Of course, refactoring is more effective as a prophylactic measure
that as a last-restort therapy. As with any edifice, it is a judgement
call, whether to rehab or restort for the wrecking ball. Another
alternative is to reassess whether new components and frameworks have
come along that can replace all or part of the system. When you can
reuse and retrofit other existing components, you can spare yourself
the time and expense involved in rebuilding, repairing, and
maintaining the one you have.

The United States Commerce Department defines durable goods as
those that are designed to last for three years or more. This category
traditionally applied to goods such as furniture, appliances,
automobiles, and business machines. Ironically, as computer equipment
is depreciating ever more quickly, it is increasingly our software
artifacts, and not our hardware, that fulfill this criterion. Firmitas
has come to the realm of bits and bytes.

Apple's Lisa Toolkit, and its successor, the Macintosh Toolbox,
constitute one of the more intriguing examples of

An architect's most
useful tools are an eraser at the drafting board, and a wrecking bar
at the site-- Frank Lloyd Wright

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The SOFTWARE
TECTONICS pattern discussed in [Foote & Yoder 1996] observes that
if incremental change is deferred indefinitely, major upheaval may be
the only alternative. [Foote & Yoder 1998a] explores the WINNING TEAM
phenomenon, whereby otherwise superior technical solutions are
overwhelmed by non-technical exigencies.

Brooks
has eloquently observed that the most dangerous system an architect
will ever design is his or her second
system [Brooks 1995]. This is the notorious second-system
effect. RECONSTRUCTION provides
an opportunity for this misplaced hubris to exercise itself, so one
must keep a wary eye open for it. Still, there are times when the best
and only way to make a system better is to throw it away and start
over. Indeed, one can do worse than to heed Brook's classic admonition
that you should "plan to throw one away, you will anyway".

Mir reenters the atmosphere over Fiji on 22 March, 2001

Conclusion

In the end, software architecture is about how we distill experience
into wisdom, and disseminate it. We think the patterns herein stand
alongside other work regarding software architecture and evolution
that we cited as we went along. Still, we do not consider these
patterns to be anti-patterns. There are good reasons that good
programmers build BIG BALLS OF MUD.
It may well be that the economics of the software world are such that
the market moves so fast that long term architectural ambitions are
foolhardy, and that expedient, slash-and-burn, disposable programming
is, in fact, a state-of-the-art strategy. The success of these
approaches, in any case, is undeniable, and seals their pattern-hood.
People build

BIG BALLS OF MUD because they work.
In many domains, they are the only things that have been shown to
work. Indeed, they work where loftier approaches have yet to
demonstrate that they can compete.

It is not our purpose to condemn BIG
BALLS OF MUD. Casual architecture is natural during the early stages
of a systems evolution. The reader must surely suspect, however,
that our hope is that we can aspire to do better. By recognizing the
forces and pressures that lead to architectural malaise, and how and
when they might be confronted, we hope to set the stage for the
emergence of truly durable artifacts that can put architects in
dominant positions for years to come. The key is to ensure that the
system, its programmers, and, indeed the entire organization, learn
about the domain, and the architectural opportunities looming within
it, as the system grows and matures.

Periods of moderate disorder are a part of the ebb and flow of
software evolution. As a master chef tolerates a messy kitchen,
developers must not be afraid to get a little mud on their shoes as
they explore new territory for the first time. Architectural insight
is not the product of master plans, but of hard won experience. The
software architects of yesteryear had little choice other than to
apply the lessons they learned in successive drafts of their systems,
since RECONSTRUCTION was often
the only practical means they had of supplanting a mediocre system
with a better one. Objects, frameworks, components, and refactoring
tools provide us with another alternative. Objects present a medium
for expressing our architectural ideas at a level between
coarse-grained applications and components and low level code.
Refactoring tools and techniques finally give us the means to
cultivate these artifacts as they evolve, and capture these insights.

The onion-domed Church of the Intercession of the Virgin on the
Moat in Moscow is one of Russia's most famous landmarks. It was built
by Tsar Ivan IV just outside of the Kremlin walls in 1552 to
commemorate Russia's victory over the Tatars at Kazan. The church is
better known by it's nickname, St. Basil's. Ivan too is better known
by his nickname "Ivan the Terrible". Legend has it that once the
cathedral was completed, Ivan, ever true to his reputation, had the
architects blinded, so that they could never build anything more
beautiful. Alas, the state of software architecture today is such that
few of us need fear for our eyesight.

We'd like to express our gratitude as well to Paolo Cantoni, Chris
Olufson, Sid Wright, John Liu, Martin Cohen, John Potter, Richard
Helm, and James Noble of
the Sydney
Patterns Group, who workshopped this paper during the late winter,
er, summer of early 1998.

John Vlissides, Neil Harrison, Hans Rohnert, James Coplien, and Ralph Johnson
provided some particularly candid, incisive and useful criticism of
some of the later drafts of the paper.